Emergency roomphysicians often assess the pupillary reflex because it is useful for gauging brain stem function. Normally, pupils react (constrict) equally. Lack of the pupillary reflex or an abnormal pupillary reflex can be caused by optic nerve damage, oculomotor nerve damage, brain death and depressant drugs, such as barbiturates. The optic nerve is responsible for the afferent limb of the pupillary reflex, or in other words, senses the incoming light. The oculomotor nerve is responsible for the efferent limb of the pupillary reflex; in other words, it drives the muscles that constrict the pupil.

Normally, each pupil should constrict with light shone into either eye. On testing each reflex for each eye, several patterns are possible.

The ipsilateral consensual reflex is lost (because light shone into the opposite eye can signal the brain, causing attempted constriction of both pupils, but constriction fails on the damaged side)

The contralateral consensual reflex is intact (because light shone into the damaged eye can still signal to the brain via the normal optic nerve, causing attempted constriction of both pupils, which works on the other side via the normal oculomotor nerve on that side)

The pupillary accommodation reflex reduces the size of the pupil when an object is close to the eye. A smaller pupil produces a sharper image on the retina.
There is also a separate accommodation reflex which changes the shape of the lens so as to focus the image on the retina.